The present invention relates generally to implantable medical devices and more particularly to implantable pacemakers and to implantable pacemakers associated with implantable cardioverters and/or defibrillators.
Traditionally, implantable cardiac pacemakers have employed R-wave detector circuitry which includes an R-wave amplifier in series with a detector circuit which detects the excursion of the amplified R-wave signal past a predetermined threshold. Adjustment of the sensitivity of the R-wave detection circuitry has been accomplished both by adjusting the gain of the R-wave amplifier and by adjusting the threshold of the detector. Normally, these adjustments are accomplished during programming of the implanted pacemaker, and the selected gain and/or threshold remains constant until reprogramming at a later date.
In the field of physiologic monitoring generally, it has been known for some time that it is desirable to employ an R-wave detector in which the detection threshold is varied in response to the amplitude of the detected R-waves. Circuitry for accomplishing this is disclosed in the book, Design of Microcomputer-Based Medical Instrumentation by W. J. Tompkins and John G. Webster, Editors, Prentice-Hall, 1981, pages 413 through 415, as well as in the article, "Reliable R-wave Detection from Ambulatory Subjects" by Thakor, et al., published in Biomedical Science Instrumentation, Volume 14, pages 67 through 72, 1978, both of which are incorporated by reference in their entireties. In these disclosed R-wave detectors, the half-wave rectified output of the R-wave amplifier is provided to a sample and hold circuit which provides an adjustable threshold proportional to the amplitude of the R-wave. This threshold gradually declines with time, returning to a base threshold level after a period of approximately ten seconds or so.
Sense amplifiers having automatic gain control are also known in the specific context of implantable pacemakers associated with cardioverters and defibrillators. U.S. Pat. No. 4,819,643 issued to Menken discloses an automatic gain control amplifier in which gain is gradually increased following sensed ventricular contractions, including both spontaneous ventricular contractions and paced ventricular contractions. This amplifier is used to provide a heart rate signal to a microprocessor, for purposes of activating an implanted defibrillator. The device also includes a cardiac pacemaker, which includes a sense amplifier as discussed above, having a gain level adjustable by means of programming. The cardiac pacemaker is provided with a hysteresis feature wherein a substantially longer interval (two seconds or greater) following a sensed ventricular contraction is provided prior to delivery of a cardiac pacing pulse, to allow the gain of the AGC sense amplifier to maximize prior to pacing.